1. Technical Field
The present invention relates in general to motor vehicles. More particularly, the invention relates to a method for operating an internal combustion engine having an exhaust-gas turbocharger. More specifically, but without restriction to the particular embodiment and/or use which is shown and described for purposes of illustration, the present invention relates to a method for setting the combustion air quantity and exhaust-gas turbocharger for an internal combustion engine.
2. Discussion
The publication DE 195 43 190 A1 discloses an exhaust-gas turbocharger, the turbine of which has a turbine geometry capable of being set variably via an adjustable guide-vane cascade. The guide-vane cascade comprises guide vanes which can be set with the aid of an actuator in such a way that the effective turbine inlet cross section of the turbine is modified. It is thereby possible to implement high exhaust-gas back pressures of differing amount in the section between the cylinder outlet and the exhaust-gas turbocharger. Therefore, the output of the turbine and the output of the compressor can be set according to current requirements.
Shut-off bodies can be moved into the interspaces between the guide vanes of the guide-vane cascade, with the result that the effective turbine inlet cross section is additionally reduced and, correspondingly, the exhaust-gas back pressure is additionally increased. The shut-off bodies are inserted into the interspaces between the guide vanes in order to reduce the inlet cross section to a minimum. Therefore, in an engine braking mode, the exhaust-gas back pressure necessary for the desired braking action is achieved in the exhaust tract.
Depending on the mode of operation and the operating state of the internal combustion engine, the rotor output of the exhaust-gas turbocharger is influenced by the corresponding setting of the variable turbine geometry. The charging pressure is maintained at a settable excess-pressure level above the ambient pressure via the rotor output.
The supply of combustion air into the inlet of the cylinders is determined not only by the charging pressure, but also by the position of the throttle valve in the intake tract. It is necessary for the modes of operation of the throttle valve and of the exhaust-gas turbocharger, in particular of the variable turbine geometry, to be coordinated with one another so that an optimum fuel/air ratio can be maintained.
A further exhaust-gas turbocharger with variable turbine geometry is known from the publication U.S. Pat. No. 4,867,637. The turbine geometry consists of a radial guide-vane cascade with guide vanes which are distributed over the circumference and can each be pivoted about their fastening axes through an angle of at most 30xc2x0 between a closing position and an opening position. The increase in cross section due to the pivoting of the guide vanes causes the exhaust-gas back pressure to be reduced and maintained at a level which does not damage the components.
The problem on which the invention is based is to provide a greater range of use for exhaust-gas turbochargers with variable turbine geometry.
According to the novel method for setting the combustion-air quantity, the direction or rotation of the rotor of the exhaust-gas turbocharger is set as a function of the current state variables and operating variables of the internal combustion engine and of the associated assemblies. It is possible, by reversing the direction of rotation of the rotor, to generate a vacuum in the cylinder inlet. In this mode of operation, the compressor functions as a throttling device, and the throttling capacity can be adapted to current requirements via the variable turbine geometry.
Depending on the position of the variable turbine geometry and on the corresponding direction of rotation of the rotor, excess pressure or a vacuum is generated in the intake tract downstream of the compressor. The generation of the vacuum makes it possible to extend the possibilities for using the exhaust-gas turbocharger in a throttling mode, in addition to the charging or compression mode already known in the art. The novel possibility for using the exhaust-gas turbocharger allows the compressor to be employed as a throttling device, so that, in principle, it is possible to dispense with a throttle valve in the intake tract and regulate the dimensioning of the air supply solely via the charger. In so doing, in addition to a simplified design, the advantage that intake pressure can be set acccurately and continuously both in the compression range and in the throttling range is achieved.
In an expedient version, use of the exhaust-gas turbocharger in the throttling range is combined with the use of a throttle valve, so that the respective best behaviour of the supercharger and throttle valve can be utilized at selected operating points. Both the fuel consumption and the dynamic behavior of the engine may be influenced beneficially thereby.
In one form, a combined use of the throttle valve and turbocharger is conducted in an advantageous manner. In the entire compression range when the intake pressure is at the excess-pressure level, the regulation of the air quantity is carried out by the turbocharger. In the throttling range when the intake pressure is at the vacuum level, but above an intake-pressure limit value, the regulation of the air quantity is carried out solely via the setting of the exhaust-gas turbocharger. In this operating range, the throttle valve remains completely open and air metering is regulated solely via the setting of the charger.
By contrast, in a lower load range below the intake-pressure limit value, to shift the guide-vane cascade position of the turbine into the opening position and carry out air metering via the setting of the throttle valve. Serious disadvantages in the transient behaviour of the exhaust-gas turbocharger in the lower load range can thereby be avoided.
The turbine of the exhaust-gas turbocharger according to the invention is equipped with a guide-vane cascade which is adjustable, in a state-dependent and operationally dependent manner, into a position reversing the direction of rotation of the rotor. One of the directions of rotation being assigned to the generation of excess pressure and the other direction of rotation to the generation of a vacuum in the cylinder inlet. In a middle position between the stop positions, the rotor is stationary, no appreciable excess pressure or vacuum is generated, and ambient pressure prevails in the intake tract downstream of the compressor. In the two stop positions, the rotor rotates in opposite directions of rotation, and, in the range between one of the stop positions and the middle position, in each case only compression excess pressure or a throttling vacuum is generated. The magnitude of the excess pressure or vacuum depends on the current position of the various turbine geometry.
In an advantageous refinement, the guide-vane cascade has adjustable guide vanes which are adjustable, in particular through up to 180xc2x0, between two opposite stop positions.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.